1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer.
2. Description of Related Art
A conventional image forming apparatus, such as a laser printer, includes a photosensitive drum and a processing unit. The processing unit includes: a charger; a scanner device; a developing roller; and a transfer roller, which are arranged around the photosensitive drum in this order in the rotating direction of the photosensitive drum. As the photosensitive drum rotates, the surface of the photosensitive drum is first charged uniformly by the charger. The photosensitive drum surface is then exposed by a laser beam, which is modulated according to prescribed image data and which is scanned by the scanner device at a high speed. As a result, an electrostatic latent image is formed on the photosensitive drum. The processing unit stores toner therein. The developing roller is supplied with toner, and a thin layer of toner is formed on the surface of the developing roller. As the developing roller rotates, toner on the developing roller confronts the photosensitive drum. As a result, toner is supplied onto the electrostatic latent image on the surface of the photosensitive drum. Toner is borne selectively on the surface of the photosensitive drum, thereby developing the electrostatic latent image into a visible toner image. Afterwardly, the visible image thus produced on the surface of the photosensitive drum confronts the transfer roller, and is transferred onto a recording sheet when the recording sheet passes between the photosensitive drum and the transfer roller.
It is conceivable to provide an image forming apparatus of a type, in which a cleaning roller is provided in the downstream side of the transfer roller in the rotational direction of the photosensitive drum. The cleaning roller is for collecting residual toner, which remains on the photosensitive drum after the toner image is transferred onto the recording sheet.
In this conceivable image forming apparatus, the cleaning roller rotates in a predetermined, fixed rotational speed while contacting with the photosensitive drum. The cleaning roller is applied with a predetermined, fixed bias voltage with respect to the photosensitive drum. After the toner image is transferred onto a recording sheet, the cleaning roller electrically collects the residual toner from the photosensitive drum, while rotatingly contacting with the photosensitive drum.
In this conceivable image forming apparatus, however, when a recording sheet is jammed in the processing unit, a larger-amount of toner will remain on the photosensitive drum. Considering this problem, it is conceivable to set the values of the rotational speed and the bias voltage of the cleaning roller to sufficiently high values so that the cleaning roller can sufficiently collect even this large-amount of toner. However, it is unnecessary to operate the cleaning roller at such a high rotational speed and with such a great bias voltage when a recording sheet is not jammed. If the cleaning roller is operated at such a high rotational speed and with such a great bias voltage also during the normal image forming process, the cleaning roller will excessively forcibly rub toner against the surface of the photosensitive drum. This will lead to filming of toner on the photosensitive drum. The image quality will be degraded, and the life of the photosensitive drum will be shortened.
In view of the above-described drawbacks, it is an objective of the present invention to solve the above-described problems and to provide an improved image forming apparatus that can properly collect a large amount of developing agent that resides on the photosensitive drum when a trouble occurs in the conveyance of the recording sheet, that maintains high image quality, and that ensures the image bearing body to have a sufficiently long life.
In order to attain the above and other objects, the present invention provides an image forming apparatus, comprising: a conveying unit; a trouble-detecting unit; a trouble-clearing-up judging unit; an image bearing body; a transfer unit; a cleaning roller; and a control unit. The conveying unit conveys a recording medium. The trouble-detecting unit detects whether some trouble occurs against conveyance of the recording medium. The trouble-clearing-up judging unit judges whether the trouble detected by the trouble-detecting unit has been cleared up. The image bearing body bears thereon a visible image, which is made from a developing agent electrically charged in a predetermined polarity. The transfer unit is located opposing the image bearing body and transfers the visible image onto the recording medium. The cleaning roller is located opposing and contacting the image bearing body and collects the developing agent from the image bearing body. The control unit applies the transfer unit with a bias voltage of a polarity opposite to that of the developing agent when the trouble-detecting unit detects no trouble. The control unit applies, when the trouble-judging unit detects some trouble, the transfer unit with a bias voltage of a polarity the same as that of the developing agent after the trouble-clearing-up judging unit judges clearing-up of the trouble.
With this configuration, the control unit applies the transfer unit with the bias voltage, whose polarity is the same as that of the developing agent after the conveyance trouble is cleared up. It is therefore possible to prevent a large amount of developing agent, which remains on the image bearing body due to the conveyance trouble, from being attached to the transfer unit. It is ensured that the large amount of developing agent be collected by the cleaning roller.
It is preferable that the control unit controls at least one of the cleaning roller and the image bearing body in a first condition when the trouble-detecting unit detects no trouble, and that the control unit controls, when the trouble-judging unit detects some trouble, the at least one of the cleaning roller and the image bearing body in a second condition that is different from the first condition after the trouble-clearing-up judging unit judges clearing-up of the trouble.
With this configuration, during the normal image forming time, at least one of the cleaning roller and the image bearing body is operated in the first operating condition. It is ensured that during the normal image forming process, the cleaning roller will not be operated in the second operating condition that corresponds to the conveyance-trouble. It is therefore possible to prevent the cleaning roller from excessively sliding against the image bearing body. It is possible to prevent image quality from being degraded due to the excessive sliding of the cleaning roller against the image bearing body. It is also possible to prevent the life of the image bearing body from being shortened due to the excessive sliding of the cleaning roller against the image bearing body.
On the other hand, when some trouble occurs against the conveyance of the recording medium, after the trouble is cleared up, the controlling unit operates at least one of the cleaning roller and the image bearing body in the second condition that is different from the first condition. Even when a large amount of developing agent remains on the image bearing body due to the conveyance trouble, the cleaning roller can properly and sufficiently collect the large amount of developing agent.
It is preferable that the control unit changes the second condition in accordance with an image-forming number indicative of the number of images which the image bearing body has produced until the trouble-detecting unit detects the trouble.
As the image-forming number, that is, the number of images formed by the image forming apparatus increases, the developing agent and the components in the apparatus will normally be degraded, and the density of images formed on the image bearing body will rise. Accordingly, the amount of the developing agent that remains on the image bearing body after the occurrence of the conveyance-trouble will increase as the image-forming number increases. According to the present invention, therefore, when the amount of the developing agent remaining on the image bearing body increases as the image-forming number increases, the control unit changes the second condition dependently on the image-forming-number. It is ensured that the cleaning roller can always collect the remaining developing agent properly and sufficiently regardless of the image-forming-number.
It is also preferable that the control unfit changes the second condition dependently on temperature and humidity in an environment where the image forming apparatus is located.
When at least one of temperature and humidity rises in the environment where the image forming apparatus is located, the density of images on the age bearing body increases. On the other hand, when at least one of temperature and humidity lowers, the image density decreases. Accordingly, when some trouble occurs in the conveyance of the recording medium, the amount of developing amount remaining on the image bearing body due to the trouble changes according to the temperature and/or humidity at the time when the trouble occurs. According to the present invention, therefore, when the amount of developing amount remaining on the image bearing body changes according to the temperature and/or humidity, the control unit changes the second condition dependently on the temperature and/or humidity. It is ensured that the cleaning roller can properly and sufficiently collect the remaining developing agent from the image bearing body regardless of the temperature or humidity.
The image forming apparatus may further comprise a charging unit electrically charging the image bearing body. In this case, the control unit may include a charging control unit controlling the charging unit to electrically charge the image bearing body if the trouble-clearing-up. Judging unit judges that it has taken a predetermined period of time or longer to clear up the trouble.
The electrically-charged amount of the developing agent reduces as time passes from when the developing agent has been charged. If it has taken the predetermined time or longer to clear up the trouble, even if the cleaning roller is applied with a proper amount of bias voltage, the cleaning roller will possibly collect the developing agent insufficiently due to the decreased charged amount of the developing agent. According to the present invention, therefore, if it has taken the predetermined time or longer to clear up the trouble, the charging control unit controls the charging unit to electrically charge the image bearing body. The charge-amount-decreased developing agent is again electrically charged, and can be properly collected by the cleaning roller.
The image forming apparatus may further comprise: a voltage applying unit applying a cleaning-bias voltage to the cleaning roller with respect to the image bearing body; and a driving unit rotating the cleaning roller at a cleaning-roller-rotational speed. In this case, the control unit may include: a setting unit that sets at least one of the cleaning-bias voltage and the cleaning-roller-rotational speed to a predetermined normal value while the trouble-detecting unit detects no trouble; and a changing unit that changes, after the trouble-clearing-up judging unit judges that the trouble is cleared up, at least one of the cleaning-bias voltage and the cleaning-roller-rotational speed into a changed value that is different from the predetermined normal value.
When some trouble occurs in the conveyance of the recording medium, the control unit changes the bias voltage applied to the cleaning roller and/or the rotational speed of the cleaning roller, after the trouble has been taken care of. Even when a large amount of developing agent remains on the image bearing body due to the conveyance-trouble, the cleaning roller can reliably collect the developing agent.
In this case, it is preferable that the setting unit sets at least one of an absolute value of the cleaning-bias voltage and the cleaning-roller-rotational speed to the predetermined normal value, the changing unit changing the at least one of the absolute value of the cleaning-bias voltage and the cleaning-roller-rotational speed into the changed value that is greater than the predetermined normal value.
Accordingly, when some trouble occurs in the conveyance of the recording medium, the bias voltage absolute value and/or the rotational speed is increased from the predetermined normal value. Accordingly, the collecting ability of the cleaning roller is enhanced, and can reliably collect the large amount of developing agent.
According to another aspect, the present invention provides an image forming apparatus, comprising: a conveying unit conveying a recording medium; a trouble-detecting unit detecting whether some trouble occurs against conveyance of the recording medium; a trouble-clearing-up judging unit judging whether the trouble detected by the trouble-detecting unit has been cleared up; an image bearing body bearing thereon a visible image, which is made from a developing agent electrically charged in a predetermined polarity; a transfer unit located opposing the image bearing body and transferring the visible image onto the recording medium; a cleaning roller located opposing and contacting the image bearing body and collecting the developing agent from the image bearing body; and a control unit controlling at least one of the cleaning roller and the image bearing body in a first condition when the trouble-detecting unit detects no trouble, the control unit controlling, when the trouble-judging unit detects some trouble, the at least one of the cleaning roller and the image bearing body in a second condition that is different from the first condition after the trouble-clearing-up judging unit judges clearing-up of the trouble.